The invention relates to a method and apparatus for determining the distance to an object that emits an IR signature.
The invention is important, in particular, when an aircraft is threatened by a self-propelled missile. Most short-range and medium-range missiles generally have an infrared homing head which is sensitive to radiation emitted by the aircraft (primarily from the engines), and operates according to a target-seeking guidance method. Such guided missiles are relatively simple to produce and can be launched from shoulder-carried weapons (so-called rocket launchers).
Since such missiles—in contrast to radar-guided missiles—passively head for their target, optical warning sensors are increasingly important in order to be able to initiate countermeasures against such approaching missiles at an appropriate and sufficient time.
German patent document DE 195 46 873 C1 discloses a method and apparatus having a passive imaging sensor which detects the approaching object in at least two wavelength ranges for which the atmospheric attenuation coefficients are different, by connecting a rotating filter upstream of the optical system of the passive warning sensor. (The filter consists of a plurality of sectors whose transmission ranges are different according to the selected wavelength ranges.) The sensor signals corresponding to the radiation power of the approaching object at the different wavelengths that are detected by the sensor, are then used to determine the distance and/or speed of the object (for example, of a missile approaching an aircraft), taking into account the difference in the atmospheric attenuation coefficients.
The method described in German patent document DE 195 46 873 C1 is based on spectrally resolved measurements at discrete wavelengths and determines the object distance by forming the ratio of two respective sensor signals in different wavelength ranges. However, this method is very inaccurate.
German patent document DE 34 14 798 A1 discloses a passive rangefinder for detecting radiation from emitting objects, in which sun glint radiation in the wavelength range below 2.9 μm is measured. Radiation is also measured in a so-called blue spike and a red wing, which are situated respectively on the short-wave and long-wave sides of an atmospheric absorption band. From these measurements, a theoretical spectrum is generated for each wavelength in the range of wavelengths of interest, which spectrum is weighted with a transmission factor that constitutes the proportion of radiation allowed to pass over the distance R taking into account absorption by atmospheric carbon dioxide and nitrogen oxide. The distance to the object is determined by comparing the theoretical and experimental results.
One object of the present invention is to provide a method and apparatus which can be used to provide data relating to an approaching object in a faster and more accurate manner than in the prior art.
This and other objects and advantages are achieved by the method according to the invention, in which the spectral intensity distribution (also referred to as the intensity distribution spectrum) of a detected object is measured in the region of an absorption structure of the atmosphere. In the measured intensity distribution spectrum, a point is determined having an extremal gradient on a flank of an intensity rise or fall, caused by the atmospheric absorption structure. The path length which is traveled by the radiation through the atmosphere (and corresponds to the distance between the detector and the object) is determined by comparison with known transmission data for the atmosphere. The temporal shift in the position is also expediently determined from corresponding distance measurements at two different points in time, and is used to calculate the relative speed between the detector and the object. In this case, the intensity distribution spectrum can be recorded with regard to the wavelength or the frequency.
In other words, the method according to the invention makes it possible to determine the spectral position of the flank of the spectral intensity distribution at different points in time using the point having a maximum gradient. This is considerably more accurate than the determination, as described in German patent document DE 195 46 873 C1, by forming the ratio of discrete measurements at different wavelengths and is largely independent of the spectral distribution of the intensity emitted by the object, since only a narrow wavelength range has to be evaluated.
The spectral intensity distribution is expediently investigated in a wavelength range which is in the region of atmospheric absorption lines, preferably in the vicinity of those absorption lines that are caused by gases whose concentration fluctuates only slightly with the climatic conditions. The wavelength range of, for example, 4-5 μm (expediently, 4.3-4.7 μm) which is in the mid-infrared on the short-wave and long-wave sides of the CO2 absorption line is particularly suitable for this purpose. It goes without saying that the spectral intensity distribution can also be investigated in a corresponding frequency range.
The apparatus according to the invention comprises input optics with an electrically tunable wavelength filter having a voltage-dependent filter characteristic curve, a passively imaging detector for detecting the radiation from the object, a first circuit for processing the detector signal, and a second circuit for generating a periodically varying control voltage for the wavelength filter. The first circuit is expediently constructed from a differentiating element, an analog/digital converter and a signal analysis processor, and the second circuit is essentially constructed from a function generator, a clock generator and a summation element. The apparatus also comprises a voltage source for generating an offset voltage for the voltage to be supplied to the wavelength filter.
The method according to the invention makes it possible to completely detect and evaluate the entire IR radiation emitted by an object at any point in time. The entire two-dimensional image field of an object to be investigated is thus detected. The apparatus according to the invention is thus used to supply all radiation focused by the lens into the image plane to spectral evaluation. In contrast to the prior art, as described in UK patent document GB 2 323 730 A for example, the entire object rather than object sections is considered in principle.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.